Many different types of plastic bag making machines are known in the art of producing plastic bags for industrial and individual consumers for many different applications (e.g. small sandwich bags and trash bags). While the present invention has a wide range of applications for the production of such products, the related art will be explained by reference to one particular class of bags i.e., polyethylene trash bags or, garbage bags and wastebasket liners of the type usually sold in boxes of folded bags or rolls of bags.
Further discussion of the history and operation of these machines can be found in U.S. Pat. No. 4,642,084 (the '084 patent) entitled "Plastic Bag Making Machine", issued to the present inventor on Feb. 10, 1987 and assigned to Custom Machinery Design, Inc. The '084 patent discloses a bag machine which includes a rotary drum with seal bars attached thereto and which includes a gear mechanism adapted for analog variation of the drum diameter between a first smaller diameter and a second larger diameter. Manual rotation of a hex nut assembly while the machine is stopped increases or decreases the drum's diameter through a series of appropriately mounted mitre gears and ring gears. Once this adjustment is made, the machine begins operation. Readjustment of the drum diameter can only be accomplished by stopping the machine to adjust the hex nut assembly.
Additionally, very small errors in drum diameter size can lead to acute problems, particularly an accumulating error in the seal to print registration distance every revolution of the drum. An error of a fraction of an inch leads to serious problems when the bag width is only several inches across and the speed of the film moving through the machine is 500-900 bags per minute. By the time the error is detected, a considerable amount of film is wasted.
The control of the spacial relationship between a repetitive print pattern on the web and the repetitive seals the machine is placing across the web is referred to as the "registration" of the seal to the print on the web. This spacial relationship may also be referred to as the "phase" between the repetitive print and seal occurrences on the web.
Similarly, the control of the spacial relationship between the repetitive seals placed across the web and the repetitive perforations the machine is placing across the web is referred to as the "registration" of the perforation to the seal on the web. This spacial relationship may also be referred to as the "phase" between the repetitive perforations and the repetitive seals across the web. The distance between a seal and a perforation is commonly called the "skirt length" of the finished bag.
Another prior art device described in U.S. Pat. No. 4,934,993 (the '993 patent), also issued to Gietman and assigned to the assignee of the present invention, allows for adjusting the drum diameter while the bag making machine is in operation. The '993 patent requires the operator to preset the drum diameter corresponding to the nominal bag length, but will correct for slight variations in the bag length. If the seal is not properly registered to the printing on the bag the diameter of the drum is temporarily increased or decreased. When the registration is correct the drum returns to the preset diameter. One disadvantage of this system is that "hunting" (the drum diameter will continually change) will occur if the average bag varies more than slightly from the preset length. Hunting will be particularly prevalent at higher speeds.
When a bag making machine such as that described in the '993 patent is used to adjust the drum diameter, any device (such as a perforator, die cutter, punching station, or folding station) on the bag making machine that processes the plastic downstream of the drum may become out of proper synchronization with the sealing process occurring in the drum while the drum is changing diameter. For example a perforator will be slightly out of synchronization causing perforation to seals registration (skirt length) to vary. According to the '993 patent the skirt length may be adjusted manually. However, by the time the error is detected and the manual correction made, a considerable amount of film may be wasted.
A bag making machine which overcomes these shortcomings would represent a considerable advancement in the art.